1. In a large saucepan, warm 3 tablespoons of the butter over medium-high heat until melted. Add the garlic and scallions, and cook, stirring, until the scallions are wilted, about 2 minutes.
2. Stir in the flour and cook, stirring, until the flour is no longer visible, about 30 seconds. Gradually pour in the broth, stirring constantly to keep the mixture smooth. Bring to a boil, then add the potato, carrots, 1/4 teaspoon of the nutmeg, and the pepper. Let the mixture return to a boil, then reduce the heat to medium-low, cover, and simmer until the potato is tender, about 15 minutes.
3. Meanwhile, in a small saucepan, combine the remaining 1 tablespoon butter with the remaining 1/2 teaspoon nutmeg, and cook over low heat until fragrant, about 1 minute. Brush the toast with the nutmeg butter and cut into small triangles.
4. Increase the heat under the broth mixture to medium and bring it to a simmer. Add the crabmeat, celery, and milk, and cook, stirring, until heated through, 3 to 4 minutes. Serve the chowder with the nutmeg croutons.